The github repositories are the actively developed code for Accellera’s reference implementation (sourceforge was made read-only when github was spun up).
That being said, github stores the active development for the reference implementation, not for the standard itself. The class reference (ie. The “Standard”), as well as the Accellera Reference Implementation are officially published on accellera.org:
The UVM 1.2 Standard: http://accellera.org/images/downloads/standards/uvm/UVM_Class_Reference_Manual_1.2.pdf
The UVM 1.2 Reference Implementation:
http://accellera.org/images/downloads/stan